[pushed] c++: block copy elision in delegating ctor

[Jason Merrill <jason@redhat.com>]

CWG2403 deals with the issue that copy elision is not possible when the
initialized object is a potentially-overlapping subobject and the
initializer is a function that returns by value.  Jonathan pointed out that
this also affects delegating constructors, which might be used to construct
a base subobject.

Tested x86_64-pc-linux-gnu, applying to trunk.

gcc/cp/ChangeLog:

	* call.cc (unsafe_return_slot_p): Return 2 for *this in a
	constructor.

gcc/testsuite/ChangeLog:

	* g++.dg/init/elide8.C: New test.
---
 gcc/cp/call.cc                     |  7 +++++++
 gcc/testsuite/g++.dg/init/elide8.C | 11 +++++++++++
 2 files changed, 18 insertions(+)
 create mode 100644 gcc/testsuite/g++.dg/init/elide8.C

diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index 370137ebd6d..d107a2814dc 100644
--- a/gcc/cp/call.cc
+++ b/gcc/cp/call.cc
@@ -9022,6 +9022,13 @@ unsafe_return_slot_p (tree t)
   if (is_empty_base_ref (t))
     return 2;
 
+  /* A delegating constructor might be used to initialize a base.  */
+  if (current_function_decl
+      && DECL_CONSTRUCTOR_P (current_function_decl)
+      && (t == current_class_ref
+	  || tree_strip_nop_conversions (t) == current_class_ptr))
+    return 2;
+
   STRIP_NOPS (t);
   if (TREE_CODE (t) == ADDR_EXPR)
     t = TREE_OPERAND (t, 0);
diff --git a/gcc/testsuite/g++.dg/init/elide8.C b/gcc/testsuite/g++.dg/init/elide8.C
new file mode 100644
index 00000000000..31f899b6623
--- /dev/null
+++ b/gcc/testsuite/g++.dg/init/elide8.C
@@ -0,0 +1,11 @@
+// CWG 2403 case 3: we can't elide this copy because the delegating constructor
+// might be used to initialize a base.
+// { dg-do compile { target c++11 } }
+
+struct Noncopyable {
+  Noncopyable() = default;
+  Noncopyable(const Noncopyable &) = delete;
+  Noncopyable(int) : Noncopyable(make()) {} // { dg-error "deleted" }
+
+  static Noncopyable make();
+};

base-commit: 30e160475489867a09ed89532cae135b5849cf98
-- 
2.31.1